Conventional fiber optic cables include optical fibers which are capable of transmitting voice, television, and computer data information. Fiber optic cables designed for indoor, outdoor, or indoor/outdoor applications may be of the monotube type. The monotube design includes a single tube which surrounds optical fibers loosely disposed in the tube. An example of a commercially available monotube type fiber optic cable is an SST-Ribbon Cable made by SIECOR Corporation of Hickory, N.C.
A fiber optic cable should have a craft-friendly construction which permits ease of installation. Installation of a fiber optic cable typically requires the pulling of a cable through a passageway. The passageways are, for example, ducts, tubes, cable enclosures, or splice boxes which may not afford much room for the pulling of the fiber optic cable. Pulling-grips have been developed to facilitate the pulling of a fiber optic cable through a passageway. A pulling-grip, such as a standard fiber optic pulling-grip made by the Lewis Manufacturing Company, includes a woven steel braid section attached to a pulling eye. In a typical operation of the pulling-grip, the craftsman places the braided section over the end of a prepared end of a fiber optic cable, and then the pulling eye is pulled into the passageway. Pulling on the pulling eye causes the woven braided section to contract about the outer jacket of the fiber optic cable, thereby gripping the jacket in a fashion similar to the action of a "Chinese finger" toy.
The ease with which a fiber optic cable is routed through a passageway is dependent on certain characteristics of the fiber optic cable. For example, craftsmen have found that contact surface area for a braided section of a pulling-grip is maximized by a round profile fiber optic cable thereby avoiding slippage or disconnection of the pulling-grip during the cable pulling operation. Additionally, a light-weight cable is generally easier to pull than a heavy cable. Cable flexibility is a factor as the use of stiff cable components makes the cable difficult to bend during the cable pulling operation. Cable size is also a factor as a cable with a small cross sectional area is generally easier to pull through a narrow passageway than a cable with a large cross sectional area. Moreover, apart from ease of installation, the potential for undesirable temperature effects and the cost per unit length of the cable may be important factors in deciding between commercially available fiber optic cables.
Taking the foregoing factors into consideration, several monotube type fiber optic cable designs comprise part of the background of the present invention. For example, a monotube fiber optic cable which may be difficult to route through a passageway is disclosed in U.S. Pat. No. 5,029,974. This known fiber optic cable includes two steel strength members embedded in a round profile outer cable jacket. The steel strength members are designed to resist axial compression due to, for example, aging shrinkage or thermal contraction of the cable jacket. Resistance to axial compression and tension prevents stress being applied to the core tube and/or buckling of the fiber optic cable. Axial compression or tension on the cable jacket may otherwise cause attenuation in, or breakage of, optical fibers in the core tube. However, the use of steel strength members creates a spark hazard and their weight may negatively effect the cable pulling operation. Additionally, the round profile jacket is formed of a relatively significant quantity of plastic material, which increases the weight, size, stiffness, and cost per unit length of the cable.
Another monotube type fiber optic cable which may be difficult to route through passageways is disclosed in U.S. Pat. No. 4,844,575. This known cable is of the composite cable type and includes a monotube with optical fibers therein, and steel strength members with adjacent electrical conductors which are surrounded by an oval profile cable jacket. The electrical conductors contribute to the weight, stiffness, and cost of the cable and may make a craftsman's access to optical fibers difficult. Furthermore, the oval profile jacket requires a relatively significant quantity of plastic material, which further increases the weight, size, stiffness, and cost of the cable. Other composite cables having non-circular profiles which may experience at least the same disadvantages are disclosed in U.S. Pat. No. 5,469,523 and U.S. Pat. No. 5,039,195.
Another fiber optic cable which may be difficult to route through enclosures is disclosed in U.S. Pat. No. 4,610,505. This known fiber optic cable includes strength members which have a thermal characteristics mismatch relative to the optical fibers therein. The strength members are surrounded by engaging members which include wires helically wrapped around the strength members. The strength members with wires are then lashed to an optical fiber cable with glass or metallic lashing wires, which lashing wires are surrounded by a non-circular profile jacket. The engaging members, strength members, helically wrapped wires, lashing wires, and non-circular profile jacket disadvantageously increase the weight, stiffness, size, and cost of the fiber optic cable and increase the difficulty of manufacturability of the cable. Furthermore, the flat sides of the noncircular profile jacket do not present an optimal amount of contact surface area with a standard pulling-grip which may result in slippage during pulling of the fiber optic cable.
Examples of fiber optic cables which include a circular profile jacket are disclosed in U.S. Pat. No. 5,109,457 and U.S. Pat. No. 5,509,097. Each one of these known fiber optic cables comprises a monotube design with non-metallic strength members. The circular profile jacket of each cable is expensive because it requires a significant quantity of plastic material. Moreover, U.S. Pat. No. 5,509,097 has large interstices adjacent to the strength members which interstices must be filled with a quantity of water blocking material thereby adding to the cost of the cable and rendering manufacturability of the cable difficult. U.S. Pat. No. 5,109,457 requires a water blocking tape adjacent the strength members thereby increasing the cost of the cable.